Certain types of electrical apparatus in electrical systems are such that they are seldom activated but must be able to be activated quickly when required. The losses of the apparatus contribute to the losses of the system. Admittedly this contribution is rather slight but the losses of the apparatus affect its cost since, in many cases, it must be water-cooled, which is expensive. An apparatus dimensioned for continuous high power also incurs high costs.
With the objective of overcoming these drawbacks it is already known to use a commutation contact to bypass these types of apparatus. The apparatus therefore need not be dimensioned for a continuous current, but only for brief surges. A high power in the apparatus can then be accepted for a short time since it automatically has a thermal buffer in the form of the masses always present. The apparatus can thus operate without water-cooling. This, together with the slimmer dimensioning, enables great savings.
Important examples of apparatus of these type are current limiters and breakers. However, the invention is not limited to these applications. Breakers based on power semiconductors are expensive and cause losses. For most of its lifetime a breaker is passively in the on position and conducts current. It is active during extremely short periods when it opens the circuit and breaks the current. In the same way it then stays in open position and later becomes active during a short period when it closes the circuit. While the breaker is in closed state and conducting current it develops power in the form of losses that must be cooled off. In open state the current is zero and the losses are thus also zero.
If a commutation contact is connected in parallel with the semiconductor breaker, the commutation contact will conduct all current when the breaker is in closed state. When the circuit is to be broken, the commutation contact opens first and commutates all current over to the semiconductor breaker. The current in the commutation contact becomes zero and it is in open position. The semiconductor breaker can now become active and break the current in the circuit.
A breaker and a current limiter have in principle the same function apart from the speed with which they break the current. A breaker breaks at the current's zero crossing whereas the current limiter intervenes earlier and breaks an extremely high current.
Similarly a commutation contact can be used for several applications involving apparatus with high losses but which are only active for brief periods. A current limiter may consist of an electric switch parallel-connected to a commutation circuit to which the current is commutated when the electric switch breaks. During normal operating conditions, thus, the current is thus permitted to flow through the electric switch without losses. In the event of a fault causing the current to increase strongly the electric switch will commutate the current over to the parallel branch. This must take place extremely fast. The stipulation for commutating current from one branch to another is that a voltage must be generated in the branch conducting the current. The amplitude of the voltage required depends on the amplitude of the current at the instant when commutation is to occur, on the impedance in the parallel branch to which the current shall be commutated and on the duration of the commutation process. The commutations process must take place fast in order to minimise power development in the commutation apparatus and thus the damages or the dimensioning of the commutation apparatus. The commutation is facilitated if it can be delayed until the natural zero crossing of the current in alternating current networks. A mechanical contact gives lower loses when it conducts current. However, the voltage it can build up when the contacts open is limited to the voltage over the arc formed between the contacts. High arc voltage is a condition for rapid commutation with a mechanical contact.